Method, program product and apparatus for performing double exposure lithography

ABSTRACT

A method of generating complementary masks based on a target pattern having features to be imaged on a substrate for use in a multiple-exposure lithographic imaging process is disclosed. The method includes defining an initial H-mask and an initial V-mask corresponding to the target pattern; identifying horizontal critical features in the H-mask and vertical critical features in the V-mask; assigning a first phase shift and a first percentage transmission to the horizontal critical features, which are to be formed in the H-mask; and assigning a second phase shift and a second percentage transmission to the vertical critical features, which are to be formed in the V-mask. The method further includes the step of assigning chrome to all non-critical features in the H-mask and the V-mask.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/691,552, filed on Jan. 21, 2010, now U.S. Pat. No. 8,122,391, whichis a continuation of U.S. patent application Ser. No. 11/402,273, filedon Apr. 12, 2006, now U.S. Pat. No. 7,681,171, which claims priority toU.S. Provisional Application No. 60/670,285, filed on Apr. 12, 2005, allof which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The technical field of the present invention relates generally to amethod, program product and apparatus for performing a double exposurelithography, which utilizes a tri-tone mask and which provides forimproved scatter bar trimming

BACKGROUND OF THE INVENTION

Lithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In such a case, the mask may contain acircuit pattern corresponding to an individual layer of the IC, and thispattern can be imaged onto a target portion (e.g., comprising one ormore dies) on a substrate (silicon wafer) that has been coated with alayer of radiation-sensitive material (resist). In general, a singlewafer will contain a whole network of adjacent target portions that aresuccessively irradiated via the projection system, one at a time. In onetype of lithographic projection apparatus, each target portion isirradiated by exposing the entire mask pattern onto the target portionin one go; such an apparatus is commonly referred to as a wafer stepper.In an alternative apparatus, commonly referred to as a step-and-scanapparatus, each target portion is irradiated by progressively scanningthe mask pattern under the projection beam in a given referencedirection (the “scanning” direction) while synchronously scanning thesubstrate table parallel or anti-parallel to this direction. Since, ingeneral, the projection system will have a magnification factor M(generally <1), the speed V at which the substrate table is scanned willbe a factor M times that at which the mask table is scanned. Moreinformation with regard to lithographic devices as described herein canbe gleaned, for example, from U.S. Pat. No. 6,046,792, incorporatedherein by reference.

In a manufacturing process using a lithographic projection apparatus, amask pattern is imaged onto a substrate that is at least partiallycovered by a layer of radiation-sensitive material (resist). Prior tothis imaging step, the substrate may undergo various procedures, such aspriming, resist coating and a soft bake. After exposure, the substratemay be subjected to other procedures, such as a post-exposure bake(PEB), development, a hard bake and measurement/inspection of the imagedfeatures. This array of procedures is used as a basis to pattern anindividual layer of a device, e.g., an IC. Such a patterned layer maythen undergo various processes such as etching, ion-implantation(doping), metallization, oxidation, chemo-mechanical polishing, etc.,all intended to finish off an individual layer. If several layers arerequired, then the whole procedure, or a variant thereof, will have tobe repeated for each new layer. Eventually, an array of devices will bepresent on the substrate (wafer). These devices are then separated fromone another by a technique such as dicing or sawing, whence theindividual devices can be mounted on a carrier, connected to pins, etc.

For the sake of simplicity, the projection system may hereinafter bereferred to as the “lens;” however, this term should be broadlyinterpreted as encompassing various types of projection systems,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens.”Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Twin stage lithographicapparatus are described, for example, in U.S. Pat. No. 5,969,441,incorporated herein by reference.

The photolithographic masks referred to above comprise geometricpatterns corresponding to the circuit components to be integrated onto asilicon wafer. The patterns used to create such masks are generatedutilizing CAD (computer-aided design) programs, this process often beingreferred to as EDA (electronic design automation). Most CAD programsfollow a set of predetermined design rules in order to create functionalmasks. These rules are set by processing and design limitations. Forexample, design rules define the space tolerance between circuit devices(such as gates, capacitors, etc.) or interconnect lines, so as to ensurethat the circuit devices or lines do not interact with one another in anundesirable way. The design rule limitations are typically referred toas “critical dimensions” (CD). A critical dimension of a circuit can bedefined as the smallest width of a line or hole or the smallest spacebetween two lines or two holes. Thus, the CD determines the overall sizeand density of the designed circuit.

Of course, one of the goals in integrated circuit fabrication is tofaithfully reproduce the original circuit design on the wafer (via themask). As the critical dimensions of the target patterns becomeincreasingly smaller, it is becoming increasingly harder to reproducethe target patterns on the wafer. However, there are known techniquesthat allow for a reduction in the minimum CD that can be imaged orreproduced in a wafer. One such technique is the double exposuretechnique wherein features in the target pattern are imaged in twoseparate exposures.

For example, one commonly known double exposure technique is dipoleillumination. In this technique, during a first exposure the verticaledges of the target pattern (i.e., features) are illuminated and thenduring a second exposure the horizontal edges of the target pattern areilluminated. As noted, by utilizing two exposures, improved imagingperformance may be obtained.

In addition, the use of scattering bars “SB” (or assist features “AF”)has become indispensable as chip manufacturers move to more aggressivedesign rules and lower k, factors in production. The width “d” of the SBcan be estimated using the following equation, where ksg is a scalingconstant to indicate non-printability or sub-resolution (a typical rangeof the SB scaling factor k_(sb) is 0.2-0.25):d=k _(sb)(λ/NA),

where λ is the wavelength of the exposure tool, and NA is the numericalaperture of the exposure tool.

In order to maintain k₁ above 0.35, manufacturers tend to use a higherNA exposure tool. With the advent of immersion lithography, the NA valuecan be made greater than 1. Under such hyper NA conditions, SBscalability and printability are becoming a critical issue. FIG. 1 aplots the allowable SB width versus the half pitch minimum design rule.The secondary axis is the k₁ factor. As indicated by FIG. 1 a, as devicemanufacturers move to lower k₁ production, the SB width needs to alsoscale down accordingly in order to avoid unwanted printing of the SBs.This presents a problem in that at some point the required width of theSB to avoid printing will be smaller than the minimum manufacturablewidth (i.e., the required SB will be too small to manufacture).

Moreover, as the SB width on reticles becomes smaller than the exposurewavelength, λ, the Kirchoff scalar theory is no longer valid. FIG. 1 billustrates a comparison of a simulated aerial image of an isolated lineusing 4.85 NA and QUASAR illumination. The SB width on 4× reticle is 60nm on a BIM (i.e., bright intensity binary mask). Referring to FIG. 1 b,comparing the aerial image for rigorous EMF (NA85QS9363rig) versusscalar (NA85QS9363sc1), the EMF aerial image indicates that the SB isactually darker than the scalar image predicted. This suggests that SBshave more optical weight and are therefore more readily/easily printableon wafers. Accordingly, there is a need for a process which addressesthe SB scalability and printability and prevents the printing of SBs.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention toprovide a double exposure lithography method which trims (i.e., removes)unwanted SB residues from the substrate, that is suitable for use, forexample, when printing 65 nm or 45 nm node devices or less.

In summary, the present invention relates to a method of generatingcomplementary masks based on a target pattern having features to beimaged on a substrate for use in a multiple-exposure lithographicimaging process. The method includes the steps of: defining an initialH-mask corresponding to the target pattern; defining an initial V-maskcorresponding to the target pattern; identifying horizontal criticalfeatures in the H-mask having a width which is less than a predeterminedcritical width; identifying vertical critical features in the V-maskhaving a width which is less than a predetermined critical width;assigning a first phase shift and a first percentage transmission to thehorizontal critical features, which are to be formed in the H-mask; andassigning a second phase shift and a second percentage transmission tothe vertical critical features, which are to be formed in the V-mask.The method further includes the step of assigning chrome to allnon-critical features in the H-mask and the V-mask. The non-criticalfeatures are those features having a width which is greater than orequal to the predetermined critical width. The non-critical features areformed in the H-mask and the V-mask utilizing chrome. The target patternis then imaged on the substrate by imaging both the H-mask and V-mask.

The present invention provides important advantages over the prior art.For example, the present invention provides the ability to utilize largeSBs due to the mutual trimming of SBs that results from the process ofthe present invention. Specifically, in the given process, both theH-mask and the V-mask contain circuit features and SBs, but they are indifferent corresponding orientations, and therefore, there is a mutualSB trimming for the H-mask and V-mask during the two exposures.

Additional advantages of the present invention will become apparent tothose skilled in the art from the following detailed description ofexemplary embodiments of the present invention.

Although specific reference may be made in this text to the use of theinvention in the manufacture of ICs, it should be explicitly understoodthat the invention has many other possible applications. For example, itmay be employed in the manufacture of integrated optical systems,guidance and detection patterns for magnetic domain memories,liquid-crystal display panels, thin-film magnetic heads, etc. Theskilled artisan will appreciate that, in the context of such alternativeapplications, any use of the terms “reticle”, “wafer” or “die” in thistext should be considered as being replaced by the more general terms“mask”, “substrate” and “target portion”, respectively.

The invention itself, together with further objects and advantages, canbe better understood by reference to the following detailed descriptionand the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a illustrates a plot of the allowable SB width versus the halfpitch minimum design rule,

FIG. 1 b illustrates a comparison of a simulated aerial image of anisolated line using 0.85 NA and QUASAR illumination.

FIG. 2 is an exemplary flowchart illustrating the DEL/DDL layoutdecomposition method of the present invention.

FIGS. 3 a-3 d illustrate an example of the decomposition of the targetpattern (see, FIG. 3 a) into an V-layout (see, FIG. 3 b) and a H-layout(see, FIG. 3 c) in accordance with the method of the present inventionand the resulting aerial image (see, FIG. 3 d).

FIGS. 4 a-4 e illustrate how it is possible to further improve imagingperformance by utilizing illumination polarization in combination withthe process set forth in the flow chart of FIG. 2.

FIGS. 5 a-5 c illustrate a simulation comparison of the performance ofdouble exposure technique of the present invention for imaging anisolated 45 nm line with varying SB width.

FIGS. 6 a and 6 b illustrate a comparison between a single exposureprocess versus the double exposure process for 45 nm dense and isolatedlines.

FIGS. 7 a and 7 b illustrate the use of the DDL/DET technique of thepresent invention with main feature trimming on the active layer of aDRAM cell.

FIGS. 8 a and 8 c illustrate an example of resulting resist contoursformed using the double exposure technique, while FIGS. 8 b and 8 dillustrate the resist contours formed using a single exposure techniquewith an optimized illuminator. FIG. 8 e illustrates the simulatedprocess latitude using 0.85 NA dry without polarization on a cut lineacross the active area CD.

FIG. 9 is a block diagram that illustrates a computer system which canbe utilized to execute the method of the present invention and generatefiles representing the H and V masks.

FIG. 10 schematically depicts an exemplary lithographic projectionapparatus suitable for use with masks designed with the aid of thedisclosed concepts.

DETAILED DESCRIPTION OF THE INVENTION

As explained in more detail below, the double exposure technique of thepresent invention decomposes the target pattern into multiple tri-tonemasks, which when illuminated provide for improved scatter bar trimmingand improved imaging performance.

More specifically, FIG. 2 is a flow-chart illustrating a firstembodiment of the present invention. Referring to FIG. 2, the first step(Step 20) in the process is to identify and read in the target patternwhich is to be imaged on the substrate (or wafer, etc.). The targetpattern can be represented in, for example, GDSII design data or anyother suitable data format. The next step (Step 22) is to convert thetarget pattern into horizontal (H) and vertical (V) layouts, andidentify critical geometries (Step 24) in both the horizontal andvertical layouts. It is noted that when separating the target designinto H and V layouts, initially the layouts are identical and correspondto the target pattern. However, as explained in further detail below,the H-layout (also referred to as the H-mask) is modified in accordancewith the present invention and functions to print the horizontal edgesof the target pattern, and the V-layout (also referred to as the V-mask)is modified in accordance with the present invention and functions toprint the vertical edges of the target pattern.

Critical geometries are those features having a width dimension lessthan some predefined amount, which can be determined by the designerbased on the imaging system being utilized and the CD tolerance of thetarget pattern. A geometries operation is utilized to specify featuresin the H-mask and V-mask which are smaller than aspecified/predetermined value to be critical. The critical value is setas a variable which can be changed depending on the design andtechnology mode being utilized. In other words, for a given process andtechnology mode (e.g., 45 nm), it becomes more difficult to properlyimage a feature having a width dimension below a certain value, which inthe given invention is referred to as the critical value. As noted, thiscritical value may vary from process to process as well as for differenttechnology modes.

It is noted that Step 24 can be accomplished using an aerial image modelor calibrated model of the imaging system to be utilized to image thetarget pattern. By utilizing such models, it is possible to simulate howa given feature of the target pattern will be imaged on the substrate,and then based on the results of the simulation, determine the CDs ofthe features and which features would qualify as critical features. Theuse of such models is well known in the art, and will not be discussedin further detail herein. It is further noted that the aerial imagemodel or calibrated model can also be utilized to convert the targetpattern into horizontal and vertical layouts. This can be accomplishedby specifying the treatment edge in the low contrast direction withrespect to the illumination dipole X or dipole Y.

Once the critical geometries are identified in both the H-layout and theV-layout, the next step (Step 26) is to assign the desired transmission(e.g., 6% transmission) and phase (e.g., 180 degrees) to each of thecritical features in both the H-layout and the V-layout utilizing, forexample, attenuated phase-shift material. It is noted that the optimaltransmission percentage can be determined utilizing, for example, atransmission tuning technique disclosed in U.S. Pat. No. 7,514,183,which is incorporated herein in its entirety. Of course, it is alsoacceptable to simply select the desired transmission to be utilized,such as 6%. Further, while 180 degrees is identified as the desiredphase shift applied to the critical features, it is also possible toutilize other degrees of phase-shift and therefore the invention shouldnot be deemed as limited to utilizing a 180 degree phase-shift.

It is noted that the transmission and phase assigned to the criticalfeatures defines how the critical features will be formed in therespective masks utilizing, for example, the appropriate attenuatedphase-shifting material or a chromeless mesa structure. It is furthernoted that in the given embodiment, the same transmission and phase areapplied to all of the critical features to minimize the complexity ofthe mask making process. However, it is also possible to assigndifferent transmissions and phases to the critical features when doingso would result in improved imaging performance. It is noted that theoptimum transmission is NA and pitch dependent.

With regard to the non-critical features in both the H-layout andV-layout (i.e., those features having a width which is greater than thewidth defining the critical geometries), these features can be imagedutilizing chrome. Of course, if desirable for any reason, suchnon-critical features could also be imaged utilizing the same phaseshifting material utilized to form the critical features.

The next step (Step 28) is to apply tentative chrome shields to thevertical edges in the H-layout and to the horizontal edges in theV-layout. The chrome shields can be applied utilizing, for example, anyof the numerous known OPC models or optical models. Once the tentativechrome shields are applied, the next step (Step 30) is to applytentative SBs to each of the H-layout and V-layout. In the H-layout, theSBs would be placed horizontally extending parallel with the horizontaledges to be imaged, and in the V-layout, the SBs would be placedvertically extending parallel with the vertical edges to be imaged.

It is noted that one of the advantages associated with the presentinvention is the ability to utilize large SBs due to the mutual trimmingof SBs that results from the process of the present invention.Specifically, in the given process, both the I-I-mask and the V-maskcontain circuit features and SBs, but they are in differentcorresponding orientations, and therefore, there is a mutual SB trimmingfor the H-mask and V-mask during the two exposures. In other words, thebackground exposure of each exposure is utilized to prevent the SBs ineach mask from printing. This process is more efficient than using adedicated SB trimming exposure for achieving very low k₁ printing.Exactly, how large the SBs can be depends in-part on the depth of focusrequirements and the given process being utilized. One method ofdetermining the maximum SBs size permitted is to run an initialsimulation(s) to determine the maximum allowable SB width that does notresult in any SB residual in the final image.

Because there were no SBs present in the respective layouts wheninitially determining the shielding requirements, in the next step (Step32), once the tentative SBs are placed in both the H-layout andV-layout, the model for applying shielding is then re-executed so thatthe optical weight of the SBs can be taken into account with respect tothe required/optimal shielding. Similarly, once the shielding isfinalized, the OPC program for applying the SBs is re-executed so thatthe optimal SB can be determined in view of the finalized shielding tobe applied to the respective masks.

Once the SBs are finalized, the last step (Step 34) is to perform finalmodel OPC along with mask manufacture rule check and verification. Ifthe respective layouts (i.e., masks) pass the rule check andverification, the process is complete and the H-layout and V-layoutrepresent the masks to be utilized in the double exposure imagingprocess.

It is noted that the foregoing process is not limited to double dipoleillumination. For example, it is also applicable to other types ofillumination such as, for example, a customized QUASAR illumination thathas a symmetrical pole in either the X (horizontal) or Y (vertical)direction, but asymmetrical from the X to Y direction.

FIGS. 3 a-3 d illustrate an example of the decomposition of the targetpattern (see, FIG. 3 a) into an V-layout (see, FIG. 3 b) and a H-layout(see, FIG. 3 c) in accordance with the method of the present inventionand the resulting aerial image (see, FIG. 3 d). Referring to FIG. 3 b,which is the V-layout, it is shown that the horizontally orientedfeatures 33 are covered with chrome shielding, and the vertical features35 to be imaged are formed of AttPSM material having 6% transmission and180 degree phase-shift. The V-layout also includes vertically disposedSBs 37. Similarly, referring to FIG. 3 c, which is the H-layout, it isshown that the vertically oriented features 35 are covered with chromeshielding, and the horizontal features 33 to be imaged are formed ofAttPSM material having 6% transmission and 180 degree phase-shift. TheH-layout also includes horizontally disposed SBs 39. It is noted thatall of the vertical features in the V-mask and all of the horizontalfeatures in the H-mask in the foregoing example are formed of AttPSMmaterial, as all of the features have been deemed critical. However, foreach feature, the overall imaging performance is obtained as a result ofthe multiple illumination process, which functions to combineillumination of a critical feature using the AttPSM material in one maskwith illumination of the same feature utilizing chrome (i.e., chromeshield) in the other mask. Furthermore, as noted above, for any featurein the target pattern having a width which exceeds the criticaldimension, it is possible to form this feature utilizing chrome so as tosimply the mask manufacturing process. FIG. 3 d illustrates thesimulated imaging result.

In one variation of the foregoing embodiment, it is possible to furtherenhance imaging performance by utilizing the double exposure process ofthe present invention in conjunction with illumination polarization.More specifically, under high NA and strong off axis illuminationconditions, when the pitch is smaller than the wavelength, the anglebetween the 0^(th) and +/−1^(st) order diffraction component is quitelarge such that the vector effect becomes significant. Linearpolarization is an effective way of enhancing the contrast and isrelatively simple to implement in exposure systems. As is shown in FIG.4 a, the incomplete interference due to TM (i.e., traverse magneticwave) can reduce the image contrast. However, as shown in FIG. 4 b, byutilizing dipole illumination in combination with a linearly polarizedsource, the undesirable TM component can be reduced, thereby improvingthe image contrast. FIG. 4 c illustrates that for a 9% AttPSM maskexposed with 0.85 NA, DX with σ_(IN)=0.68 and σ_(OUT)=0.93, there is athree times improvement of NILS by applying linear y polarization to theDX source.

FIG. 4 d is an exposure-defocus (ED) plot with and without polarization.It is noted that since linear y-polarized DX has a lower minimumintensity, the ED window has a higher center dose. FIG. 4 e illustratesthat with the same dipole angle and sigma parameters, by adding linear ypolarization, there is an improvement in exposure latitude of 51% forvertical features.

As noted above, the double exposure technique of the present inventionprovides improved performance over previously known techniques. FIG. 5 ais a simulation comparison of the performance of double exposuretechnique of the present invention for imaging an isolated 45 nm lineassuming a 0.93 NA dry exposure system with linear y and x polarizationfor the DX and DY illuminations, respectively. The DX and DY dipolesettings are both σ_(IN)=0.68 and σ_(out)=0.93 with a 35 degree poleangle. FIG. 5 b illustrates three pairs of SBs placed at the optimumposition with their widths varying from 25 nm in FIG. 5 a to 50 nm inFIG. 5 c. The simulated FEM plot shows that the full size SBs provide100% DOF improvement as compared to the 25 nm SB.

As a practical matter, in order to use ArF to print 45 nm features, itis expected that immersion and polarization will be utilized to improveDOF. FIGS. 6 a and 6 b compare a single exposure process versus thedouble exposure process for 45 rim dense and isolated lines. Thesimulation setting is 0.93 NA, σ_(IN)=0.76 and σ_(out)=0.96 for DXsingle exposure. In the case of double exposure, both DX and DY utilizethe same NA and illumination settings. Prolith v9.01 EMF 1 was utilizedfor all simulations, which included single exposure examples: Case A(dense), Case C (isolated) and Case E (isolated), where the dense pitchis 120 nm, and double exposure examples: Case B (dense) and Case D(isolated). Cases A and B compare the dense line performance for singleexposure and double exposure in accordance with the present invention.As shown, with proper shielding, the double exposure has the sameprocess window as the single exposure. For isolated lines with singleexposure, Case E, the process window is quite limited. However, Case Dutilizing full size 55 nm SB and the double exposure technique shows asignificant improvement in DOF.

With regard to another advantage provided by the present invention, itis noted that for some very dense device configurations, printing verysmall spaces with good control between line ends is necessary. Theprocess of the present invention provides improved process latitude whenattempting to print such small spaces. As an example, the DDL/DETtechnique of the present invention was utilized to print a 6 F² DRAMcore, and the resulting process latitude was compared to that obtainedutilizing a single exposure process. FIG. 7 a illustrates the use of theDDL/DET technique of the present invention with main feature trimming onthe active layer of a DRAM cell, where k₁ is 0.28. In this example, twodipole models with 90 degree in DX and DY, respectively, were utilizedfor converting the original target layout 71 into a 9% AttPSM corepattern 73, and also a BIM trim mask 75 that is utilized to clear theconnected resist lines into two separated line ends. The simulatedimaging result 77 is also illustrated in FIG. 7 a. FIG. 7 b illustratesthe example DEL with the main feature trimming on the same cell but witha more aggressive k₁ of 0.27. In this case, the optimum source for thefirst exposure is an X-Y asymmetrical QUASAR, and the source for thesecond exposure is a dipole with a wider σ_(in) than σ_(OUT). FIGS. 8 aand 8 c illustrate the resulting resist contours formed using the doubleexposure technique. FIGS. 8 b and 8 d illustrate the resist contoursformed using a single exposure technique with an optimized illuminator.As shown in the figures, in the single exposure case, there is a severeline-end pull-back that requires very aggressive OPC correction due topoor NILS in the line-end. It is quite difficult for the single exposureprocess to achieve the same print results as the double exposureprocess. FIG. 8 e illustrates the simulated process latitude using 0.85NA dry without polarization on a cut line across the active area CD. Asshown, for 0.18 um DOF, the double exposure process results in a 33%greater exposure latitude as compared to the optimum single exposure.

Variations of the foregoing exemplary embodiments of the presentinvention are also possible. For example, as already noted above, thedouble exposure technique of the present invention is not limited todipole illumination. Different source shapes can be utilized in the twostep illumination process. Moreover, the shape of the illuminationsource for the first illumination may be different from the shape of theillumination source for the second shape.

It is also noted that in an effort to further enhance the imagingresults, the SBs are positioned with the masks so as to best match thefrequency of the target pattern. Accordingly, in the V-mask, thevertical SBs are positioned so as to best match the frequency of thevertical features to be imaged, and in the H-mask, the horizontal SBsare positioned so as to best match the frequency of the horizontalfeatures to be imaged.

In another variation, it is possible to assign a different phase shiftand percentage transmission to the critical features in the H-mask andthe V-mask. In addition, it is also possible to assign different phaseshifts and percentage transmissions to critical in the same mask (i.e.,H-mask or V-mask). For example, features having different widths, all ofwhich are less than the critical dimension, may exhibit improved imagingperformance utilizing different phase shifts and/or differenttransmissions.

In yet another variation, sub-resolution grating blocks are added to theopen areas that contain no design features (including SBs) so as toreduce the impact of flare from the imaging system.

FIG. 9 is a block diagram that illustrates a computer system 100 whichcan implement the illumination optimization explained above. Computersystem 100 includes a bus 102 or other communication mechanism forcommunicating information, and a processor 104 coupled with bus 102 forprocessing information. Computer system 100 also includes a main memory106, such as a random access memory (RAM) or other dynamic storagedevice, coupled to bus 102 for storing information and instructions tobe executed by processor 104. Main memory 106 also may be used forstoring temporary variables or other intermediate information duringexecution of instructions to be executed by processor 104. Computersystem 100 further includes a read only memory (ROM) 108 or other staticstorage device coupled to bus 102 for storing static information andinstructions for processor 104. A storage device 110, such as a magneticdisk or optical disk, is provided and coupled to bus 102 for storinginformation and instructions.

Computer system 100 may be coupled via bus 102 to a display 112, such asa cathode ray tube (CRT) or flat panel or touch panel display fordisplaying information to a computer user. An input device 114,including alphanumeric and other keys, is coupled to bus 102 forcommunicating information and command selections to processor 104.Another type of user input device is cursor control 116, such as amouse, a trackball, or cursor direction keys for communicating directioninformation and command selections to processor 104 and for controllingcursor movement on display 112. This input device typically has twodegrees of freedom in two axes, a first axis (e.g., x) and a second axis(e.g., y), that allows the device to specify positions in a plane. Atouch panel (screen) display may also be used as an input device.

According to one embodiment of the invention, the decomposition processmay be performed by computer system 100 in response to processor 104executing one or more sequences of one or more instructions contained inmain memory 106. Such instructions may be read into main memory 106 fromanother computer-readable medium, such as storage device 110. Executionof the sequences of instructions contained in main memory 106 causesprocessor 104 to perform the process steps described herein. One or moreprocessors in a multi-processing arrangement may also be employed toexecute the sequences of instructions contained in main memory 106. Inalternative embodiments, hard-wired circuitry may be used in place of orin combination with software instructions to implement the invention.Thus, embodiments of the invention are not limited to any specificcombination of hardware circuitry and software.

The term “computer-readable medium” as used herein refers to any mediumthat participates in providing instructions to processor 104 forexecution. Such a medium may take many forms, including but not limitedto, non-volatile media, volatile media, and transmission media.Non-volatile media include, for example, optical or magnetic disks, suchas storage device 110. Volatile media include dynamic memory, such asmain memory 106. Transmission media include coaxial cables, copper wireand fiber optics, including the wires that comprise bus 102.Transmission media can also take the form of acoustic or light waves,such as those generated during radio frequency (RF) and infrared (IR)data communications. Common forms of computer-readable media include,for example, a floppy disk, a flexible disk, hard disk, magnetic tape,any other magnetic medium, a CD-ROM, DVD, any other optical medium,punch cards, paper tape, any other physical medium with patterns ofholes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip orcartridge, a carrier wave as described hereinafter, or any other mediumfrom which a computer can read.

Various forms of computer readable media may be involved in carrying oneor more sequences of one or more instructions to processor 104 forexecution. For example, the instructions may initially be borne on amagnetic disk of a remote computer. The remote computer can load theinstructions into its dynamic memory and send the instructions over atelephone line using a modem. A modem local to computer system 100 canreceive the data on the telephone line and use an infrared transmitterto convert the data to an infrared signal. An infrared detector coupledto bus 102 can receive the data carried in the infrared signal and placethe data on bus 102. Bus 102 carries the data to main memory 106, fromwhich processor 104 retrieves and executes the instructions. Theinstructions received by main memory 106 may optionally be stored onstorage device 110 either before or after execution by processor 104.

Computer system 100 also preferably includes a communication interface118 coupled to bus 102. Communication interface 118 provides a two-waydata communication coupling to a network link 120 that is connected to alocal network 122. For example, communication interface 118 may be anintegrated services digital network (ISDN) card or a modem to provide adata communication connection to a corresponding type of telephone line.As another example, communication interface 118 may be a local areanetwork (LAN) card to provide a data communication connection to acompatible LAN. Wireless links may also be implemented. In any suchimplementation, communication interface 118 sends and receiveselectrical, electromagnetic or optical signals that carry digital datastreams representing various types of information.

Network link 120 typically provides data communication through one ormore networks to other data devices. For example, network link 120 mayprovide a connection through local network 122 to a host computer 124 orto data equipment operated by an Internet Service Provider (ISP) 126.ISP 126 in turn provides data communication services through theworldwide packet data communication network, now commonly referred to asthe “Internet” 128. Local network 122 and Internet 128 both useelectrical, electromagnetic or optical signals that carry digital datastreams. The signals through the various networks and the signals onnetwork link 120 and through communication interface 118, which carrythe digital data to and from computer system 100, are exemplary forms ofcarrier waves transporting the information.

Computer system 100 can send messages and receive data, includingprogram code, through the network(s), network link 120, andcommunication interface 118. In the Internet example, a server 130 mighttransmit a requested code for an application program through Internet128, ISP 126, local network 122 and communication interface 118. Inaccordance with the invention, one such downloaded application providesfor the illumination optimization of the embodiment, for example. Thereceived code may be executed by processor 104 as it is received, and/orstored in storage device 110, or other non-volatile storage for laterexecution. In this manner, computer system 100 may obtain applicationcode in the form of a carrier wave.

FIG. 10 schematically depicts a lithographic projection apparatussuitable for use with a mask designed with the aid of the currentinvention. The apparatus comprises:

a radiation system Ex, IL, for supplying a projection beam PB ofradiation. In this particular case, the radiation system also comprisesa radiation source LA;

a first object table (mask table) MT provided with a mask holder forholding a mask MA (e.g., a reticle), and connected to first positioningmeans for accurately positioning the mask with respect to item PL;

a second object table (substrate table) WT provided with a substrateholder for holding a substrate W (e.g., a resist-coated silicon wafer),and connected to second positioning means for accurately positioning thesubstrate with respect to item PL;

a projection system (“lens”) PL (e.g., a refractive, catoptric orcatadioptric optical system) for imaging an irradiated portion of themask MA onto a target portion C (e.g., comprising one or more dies) ofthe substrate W.

As depicted herein, the apparatus is of a transmissive type (i.e., has atransmissive mask). However, in general, it may also be of a reflectivetype, for example (with a reflective mask). Alternatively, the apparatusmay employ another kind of patterning means as an alternative to the useof a mask; examples include a programmable mirror array or LCD matrix.

The source LA (e.g., a mercury lamp or excimer laser) produces a beam ofradiation. This beam is fed into an illumination system (illuminator)IL, either directly or after having traversed conditioning means, suchas a beam expander Ex, for example. The illuminator IL may compriseadjusting means AM for setting the outer and/or inner radial extent(commonly referred to as σ-outer and σ-inner, respectively) of theintensity distribution in the beam. In addition, it will generallycomprise various other components, such as an integrator IN and acondenser CO. In this way, the beam PB impinging on the mask MA has adesired uniformity and intensity distribution in its cross-section.

t should be noted with regard to FIG. 10 that the source LA may bewithin the housing of the lithographic projection apparatus (as is oftenthe case when the source LA is a mercury lamp, for example), but that itmay also be remote from the lithographic projection apparatus, theradiation beam that it produces being led into the apparatus (e.g., withthe aid of suitable directing mirrors); this latter scenario is oftenthe case when the source LA is an excimer laser (e.g., based on KrF, ArFor F₂ lasing). The current invention encompasses both of thesescenarios.

The beam PB subsequently intercepts the mask MA, which is held on a masktable MT. Having traversed the mask MA, the beam PB passes through thelens PL, which focuses the beam PB onto a target portion C of thesubstrate W. With the aid of the second positioning means (andinterferometric measuring means IF), the substrate table WT can be movedaccurately, e.g., so as to position different target portions C in thepath of the beam PB. Similarly, the first positioning means can be usedto accurately position the mask MA with respect to the path of the beamPB, e.g., after mechanical retrieval of the mask MA from a mask library,or during a scan. In general, movement of the object tables MT, WT willbe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which are not explicitlydepicted in FIG. 10. However, in the case of a wafer stepper (as opposedto a step-and-scan tool) the mask table MT may just be connected to ashort-stroke actuator, or may be fixed.

The depicted tool can be used in two different modes:

In step mode, the mask table MT is kept essentially stationary, and anentire mask image is projected in one go (i.e., a single “flash”) onto atarget portion C. The substrate table WT is then shifted in the x and/ory directions so that a different target portion C can be irradiated bythe beam PB;

In scan mode, essentially the same scenario applies, except that a giventarget portion C is not exposed in a single “flash”. Instead, the masktable MT is movable in a given direction (the so-called “scandirection”, e.g., the y direction) with a speed v, so that theprojection beam PB is caused to scan over a mask image; concurrently,the substrate table WT is simultaneously moved in the same or oppositedirection at a speed V=Mv, in which M is the magnification of the lensPL (typically, M=¼ or ⅕). In this manner, a relatively large targetportion C can be exposed, without having to compromise on resolution.

Although the present invention has been described and illustrated indetail, it is to be clearly understood that the same is by way ofillustration and example only and is not to be taken by way oflimitation, the scope of the present invention being limited only by theterms of the appended claims.

What is claimed is:
 1. A computer-implemented method, comprising: identifying a target pattern having features to be printed in a single layer of an integrated circuit device; defining data for a first mask for use with a first exposure in a multiple-exposure lithographic imaging process; defining data for a second mask for use with a second exposure in the multiple-exposure lithographic imaging process; modifying the data for the first and second masks for respectively causing portions of the target pattern to be imaged during the first and second exposures; determining a critical dimension value for features based on certain aspects of the multiple-exposure lithographic imaging process; identifying first and second critical features in the modified first and second masks that have dimensions less than the determined critical dimension value; determining first and second properties of transmissive mask material to be included in the modified first and second masks, respectively, for the first and second critical features; determining, using lithographic models and a computer, an amount of chrome shielding to include in the modified first and second masks; and determining, using image simulation by the computer based on the modified first and second masks, a size of scattering bars that can be included in the first and second masks, wherein the image simulation takes into account mutual trimming by a background exposure of the first and second exposures.
 2. The computer-implemented method of claim 1, wherein modifying includes: modifying the data for first mask to cause first edges in the target pattern to be imaged; and modifying the data for the second mask to cause second edges in the target pattern to be imaged, wherein the first and second edges extend in substantially different directions.
 3. The computer-implemented method of claim 2, wherein the substantially different directions are orthogonal to each other.
 4. The computer-implemented method of claim 2, wherein determining the amount of chrome shielding includes considering placement of the chrome shielding in positions corresponding to the first and second edges in the first and second masks, respectively.
 5. The computer-implemented method of claim 1, wherein determining the first and second properties includes determining a transmission percentage of the transmissive mask material.
 6. The computer-implemented method of claim 1, wherein determining the first and second properties includes determining a phase shift of the transmissive mask material.
 7. The computer-implemented method of claim 1, wherein the first and second properties are different.
 8. The computer-implemented method of claim 1, wherein the first and second properties are the same for all identified critical features in the first and second masks, respectively.
 9. The computer-implemented method of claim 1, wherein the first properties include first values for certain of the identified critical features in the first mask, and wherein the properties include second different values for certain other of the identified features in the first mask.
 10. The computer-implemented method of claim 1, further comprising: determining, using the image simulation, first and second sets of final scattering bars to include in the modified first and second masks, respectively, wherein determining takes into account the first and second sets of final chrome shields included in the modified first and second masks.
 11. The computer-implemented method of claim 1, wherein determining the size of the scattering bars includes determining a maximum size of the scattering bars that can be included in the first and second masks that does not result in any residual scattering bar feature appearing in the target pattern imaged during the first and second exposures due to the mutual trimming by the background exposure.
 12. The computer-implemented method of claim 11, wherein the mutual trimming accounted for by the image simulation is used to determine the maximum size of the scattering bars.
 13. The computer-implemented method of claim 11, wherein the determined maximum size of the scattering bars is a width, and wherein scattering bars included in the first mask have a substantially orthogonal orientation with respect to an orientation of the scattering bars included in the second mask. 